ORIGINAL: Marty O
Sure is hard to get fuel to run against positive air pressure (about 10-20 lbs)
Sorry chief, but those "pitots" are poorly designed to be able to guarantee positive pressure into the tank.
Assuming a perfectly designed pitot vent, if you are getting 10 psi into the tank as you indicate, then the Firebird is indeed quite a plane, as it needs to be doing 1100ft/sec or about 750 mph to get that kind of dynamic pressure from a pitot. As it is, if the Firebird is doing a relistic but quick 300 mph, the dynamic pressure available in reality would be about 1.5 psi. I'd like to see the pressure measurements where you get your figures from.[>:]
Now, that's assuming you have a well functioning pitot system. However, the pitot vents I've seen on the Firebird are located much too close to the leading edge, and are smoothly faired - like a bullet, which will contribute to the opening easily being subject to suction forces when the wing is at an increased angle of attack. A much better place for the pitot is either on a long boom- like a test airplane, or spaced away from the lower wing skin like many GA airplanes, using the wing itself to keep airflow straight into the tube. And you don't want any fairing around the tube to change the local direction of the air. As designed, the firebird vent does not encourage flow straight into the tube when the airplane is at an angle of attack. Leading-edge suction is a known phenomena which I refered to here, last time I was in a discussion like this:
http://www.rcuniverse.com/forum/m_73.../tm.htm#734222
Not only will you find further expanation from me there, but also admission by a firebird owner that it is indeed fuel venting. Could it still be possible to be seeing actual vapor formation, sure. However, there is no physical reason why fuel would not be sucked out of these vents either. Plus, the pump effect mentioned by kelly would be just miniscule, and not show any effect in such a short time. And with an actual owner of one of these planes saying it's fuel - I can easily take that explanation.
BTW - if there was someone around here to spread a "BVM" rumor, it sure wouldn't be me.